1. Field of the Invention
The present invention relates to an element substrate that is used in an inkjet printhead or the like and has an electrothermal transducer that produces discharge energy, a switching element for driving the electrothermal transducer and a logic circuit that controls the switching element, and to a printhead having such an element substrate, a head cartridge and a printing apparatus.
2. Description of the Related Art
An inkjet printhead that utilizes thermal energy to discharge ink drops is able to realize a high density multi-nozzle configuration relatively easily, thereby enabling high resolution, high quality and high speed printing. One known method of discharging ink using this type of thermal energy is a side-shooter printhead that discharges ink drops vertically upwards of a surface on which an electrothermal transducer that produces thermal energy is formed. Generally, with this type of printhead, ink for discharging is supplied from the underside of an element substrate provided with the electrothermal transducer via an ink supply port that passes through the element substrate.
An element substrate mounted on a common inkjet printhead will now be described. For illustrative purposes, the printhead of a printing apparatus is used as a terminal for various types of output. Moreover, an electrothermal transducer, an element that switches this electrothermal transducer between drive or non-drive (hereinafter, switching element), and a circuit for driving the switching element have been mounted on the same substrate. However, this configuration of inkjet is exemplary in nature and is not intended.
FIG. 6 is a schematic cross-sectional view showing part of an element substrate for a conventional printhead. Reference numeral 1 denotes a p-type semiconductor substrate composed of single crystal silicon. Reference numeral 112 denotes a p-type well region, 8 denotes an n-type drain region, 116 denotes an n-type field relaxation drain region, 7 denotes an n-type source region, and 4 denotes a gate electrode. These form a switching element that uses a metal insulator semiconductor (MIS) field-effect transistor 130. Reference numeral 117 denotes a silicon oxide layer as a thermal storage layer and an insulating layer, 141 denotes a tantalum nitride film as an electrothermal transducer, 154 denotes an aluminum alloy film as wiring, and 120 denotes a silicon nitride film as a protective layer. The above form a substrate 152 of the printhead. Here, reference numeral 150 denotes a heat producing portion, and ink is discharged from an ink discharging portion 153. A top plate 156 forms a liquid channel 155 in cooperation with the substrate 152.
Incidentally, there is increased demand for faster driving, greater energy efficiency, higher integration, lower cost, and higher performance with respect to products in recent years. A configuration is thus known in which a plurality of MIS field-effect transistors 130 utilized as switching elements such as shown in FIG. 6 are built into the semiconductor substrate 1, and the electrothermal transducer is driven by operating one of these MIS field-effect transistors 130 alone or operating a plurality of them simultaneously.
However, while a large current flows in order to drive the electrothermal transducer, leakage current sometimes occurred due to the pn reverse bias junction between the drain and the well not being able to withstand the high electric field when the conventional MIS field-effect transistors 130 are operated. In such cases, the voltage proof required of a switching element could not be satisfied. Further, when an MIS field-effect transistor utilized as a switching element has a large on-resistance, the current required to drive the electrothermal transducer is reduced as a result of wasted current consumption.
In view of this, an MIS field-effect transistor 20 such as shown in FIG. 7 is conceivable in order to solve the problem of voltage proof. The structure of the MIS field-effect transistor 20 shown in FIG. 7 differs from a normal structure, with part of an n-type well region 2 in a p-type semiconductor 1 formed as a drain by enclosing the n-type source region 7 with an p-type base region 5. This is called a double diffused metal oxide semiconductor (DMOS) transistor. The drain determining the voltage proof can thus be built in deep and at low density by building a channel into the drain utilizing the n-type well region 2, thereby enabling the voltage proof problem to be solved.
Next, a level shift circuit utilized in a driver IC for driving the electrothermal transducers will be described. The method of transmitting the drive signals of the driver IC will be described using FIG. 8. First, an input signal of 5.0V to 3.3V, for example, is input to the element substrate as a high level. This signal is transmitted to a desired bit by a decoder. This signal then passes through a source inverter circuit with a CMOS configuration, and is input to the gate of a MOS transistor utilized as a switching element.
As shown in FIGS. 8 and 10, VDD denotes a power supply line input to an AND circuit and VSS denotes a terminal wired for a ground.
It is noted that a predetermined voltage VHT provided to the CMOS inverter circuit. The voltage VHT is set so that the on-resistance during the MOS transistor drive is minimized, since minimizing the on-resistance of the MOS transistor enables the size of the MOS transistor utilized as a switching element to be minimized.
The voltage level thus generally needs to be transformed in the driver IC. A level shift circuit that connects a plurality of diodes DIODE1, DIODE 2, . . . DIODEn in series in the forward direction as shown in FIG. 9 is given as an exemplary level shift circuit for transforming the voltage level. While there are also methods of thus obtaining a desired constant voltage, multiplying the characteristic variation of one diode gives the total variation. Further, the diodes need to be large in order to prevent current-dependent voltage fluctuation. Therefore, this cannot be considered a realizable method.
In view of this, a level shift circuit that obtains a desired constant voltage by interposing a source follower transistor is given as a level shift circuit generally used. FIG. 10 shows the configuration of a circuit that interposes a source follower transistor in the circuit shown in FIG. 8.
Assume that in the circuit shown in FIG. 10, a drain voltage VH for driving the MOS transistor utilized as a switching element is 30V, VGNDH is 0V, and a gate voltage VHT is 12V. In this case, a −12V back gate voltage is applied to the source follower transistor utilized as a level shift element, and we know that a drain-source voltage proof of at least 18V is required.
FIG. 11 is a top view showing the arrangement of elements on an element substrate 101 for an inkjet printhead. Switching elements 41 and electrothermal transducers 103 having the configurations shown in FIG. 7, and a level shift circuit 49 that includes a level shift element such as shown in FIG. 10 are formed on the element substrate 101. A plurality of pads (terminals) 104, a level shift circuit input voltage pad 105 utilized for receiving supply of input voltages for the level shift circuit 49 and drive signals for the switching elements 41 from an external source, and an ink supply port forming portion 107 are also formed on the element substrate 101.
A plurality of electrothermal transducers 103 (such as 256 quantity, for example) constituting nozzles are provided in two rows over an interval of 1200 dpi (dots per inch) with the ink supply port forming portion 107 sandwiched therebetween. Ink channels (not shown) are formed on the ink supply port forming portion 107 and the electrothermal transducers 103. The element substrate 101 is combined with a top plate (not shown), and ink discharge orifices are formed in the top plate at positions corresponding to the electrothermal transducers 103. Heating the electrothermal transducers 103 by applying a voltage thereto causes ink on the electrothermal transducers 103 to foam and be discharged from the discharge orifices as a result of this energy.
FIG. 12 is a top view showing the arrangement of elements on an element substrate 101 formed with more nozzles than the element substrate of FIG. 11. In the example shown in FIG. 12, at least 512 electrothermal transducers 103 are provided over an interval of 1200 dpi, with two level shift circuits 49 being provided to accommodate this.
FIG. 13 is a circuit diagram showing a detailed configuration of a circuit configured on an element substrate such as shown in FIG. 11 or 12. Reference numeral 41 denotes a switching element, 49 denotes a level shift circuit, 50 denotes a logic gate array, and 52 denotes a level converter. The switching element 41, the logic gate array 50, the level converter 52 and a latch circuit are respectively disposed in parallel on a single chip.
Although a plurality of level converters are provided in relation to the switching elements, and one level shift circuit is provided in relation to a plurality of switching elements on the actual element substrate, one each of both the level converter and the level shift circuit are shown here.
Digital image signals input from the DATA terminal are rearranged in parallel by a shift register, and then latched with the latch circuit. When the logic gate is enabled, the switching elements 41 are turned on or off according to the signals latched in the latch circuit, and current flows to selected electrothermal transducers.
It is noted that the DMOS transistor shown in FIG. 7 is suitably used as the above switching elements.
Incidentally, there is an element substrate for a inkjet printhead that enables high precision reading of element substrate temperature by building a temperature sensor into the element substrate, as disclosed in Japanese Patent Publication Laid-open No. H2-258266. This temperature sensor is applied when controlling the ink discharge characteristics. Further, it is also known to apply the temperature sensor in cases such where a sequence is forcibly interrupted using a monitor value of the temperature sensor when an abnormality of some description occurs on the substrate, such as a power short circuit, causing the substrate temperature to be abnormally high.
U.S. Pat. No. 6,439,680 discloses an example in which a prescribed voltage generation circuit is provided in the case where noise occurs in an input voltage from an external source supplied to the head, such as a heater application voltage or the like, for example, or where a drop in the input voltage occurs. Since the output voltage is maintained substantially constant by the prescribed voltage generation circuit, a heater application voltage with little fluctuation relative to the noise input or the external voltage drop can be applied to the heaters.
The number of nozzles constituting printheads had been increasing year by year in response to high speed, high quality printing in recent years. There have tended to be further increases in the number of ink supply ports provided on a single element substrate in order to cope with multi-color inks. At the same time, the number of level shift circuits themselves has to be increased if there is an increase in the number of nozzles driven simultaneously, given that the level shift circuits supply power to the switching elements for switching the electrothermal transducers. On the other hand, despite the number of nozzles tending to increase as described above, there is greater demand for energy efficiency and cost reduction. That is, element substrate miniaturization and on-resistance reduction is ongoing. By utilizing DMOS transistors as switching elements, the current is reduced using a high voltage drive that takes advantage of the characteristics of high voltage proof and the like to realize energy savings and cost reductions, and to also achieve miniaturization.
On the other hand, similarly in relation to logic circuits utilized in cases such as where a specific electrothermal transducer is selected from a plurality of electrothermal transducers, advances are being made in high densification to cope with high speed, high quality printing at low cost. At the same time, advances are now also being made in voltage reduction from the viewpoint of energy efficiency. In the case where a voltage is not applied to a logic circuit because the power supply that applies the voltage has failed for some reason, the logic of the logic circuit becomes unstable, creating the possibility of unnecessary voltages being applied to the electrothermal transducers or switching elements. When this happens, the element substrate may also not function normally due to the logic of the element substrate getting out of control, resulting in abnormal printing or the like.
The foregoing U.S. Pat. No. 6,439,680 discloses a prescribed voltage application circuit that is provided on wiring that directly connects the heater with the input terminal to the heater. This configuration expressly requires space for providing a prescribed voltage application circuit on the head.